The present invention relates generally to ceramics, and, more particularly, to a method and apparatus for containing and directing a flowable superconducting slurry.
The history of superconductivity begins in the early twentieth century. The phenomenon of superconductivity was discovered in 1911 by Heike Onnes as part of an investigation of the physical properties of mercury at very low temperatures. In 1946, Ogg observed superconductivity in very low temperature metal-ammonia solutions. In the early 1970""s, alloys of niobium metal were found to be superconductive at liquid helium temperatures. In 1986, Bednorz and Miller observed superconductivity in a Laxe2x80x94Baxe2x80x94Cu ceramic oxide lattice at about 35 K. Shortly thereafter, a research team lead by C. W. xe2x80x9cPaulxe2x80x9d Chu announced the first material to superconduct above the liquid nitrogen threshold, a ceramic oxide having the general formula Ba2YCu3O7xe2x88x92x. Since 1988, a number of materials that exhibit superconductivity above the liquid nitrogen temperature have been identified, including various Baxe2x80x94Srxe2x80x94Caxe2x80x94Cu oxide compositions and Thxe2x80x94Caxe2x80x94Baxe2x80x94Cu oxide compositions.
Superconductors having critical temperatures (Tc""sxe2x80x94temperatures below which they behave as superconductors) above the temperature of liquid nitrogen (about 78 K at standard pressures) are very advantageous since the costs of cooling with liquid nitrogen are much less than the costs of cooling with liquid helium. Moreover, liquid nitrogen cooling systems are safer, less complicated, and less hazardous than liquid helium cooling systems.
Superconductors made from ceramic oxides also share several disadvantages. One such disadvantage of oxide ceramic superconductors is the requirement of very high purity raw materials. Ceramic oxide superconductors are extremely sensitive to impurities in the parts-per-billion range, which tend to form local non-superconducting regions within single superconducting oxide grains, degrading or destroying the superconductivity thereof. This make is extremely difficult to produce oxide superconducting powders having consistent grain-to-grain properties, and even more difficult to form bodies having consistent intra-body and/or extra-body superconductor properties.
Another disadvantage of ceramic oxide superconductors is their extreme sensitivity to slight variations in their processing environment. Slight differences in furnace temperature and/or oxygen partial pressure during annealing can result in different electrical properties (such as Tc, magnetic threshold Hc, and the like) in pieces formed from the same superconducting oxide batch. Ceramic superconducting oxides are especially sensitive to oxygen partial pressure during processing, since most tend to have an oxygen-deficient perovskite structure. In other words, ceramic superconducting oxides such as Ba2YCU3O7xe2x88x92x, are non-stoichiometric permutations of the stoichiometric perovskite-structured composition Ba2YCU3O9, wherein nearly ⅓ of the oxygen atoms have been removed. As a result, the material is very sensitive to the variations in processing occurring during the critical oxygenation step.
Still another disadvantage inherent in ceramic oxide superconductors is that they are relatively brittle. Even the xe2x80x9cflexiblexe2x80x9d thin films or wires formed from oxide superconductor compositions are relatively brittle as compared to traditional metal wires.
Yet another disadvantage of ceramic oxide superconductors is that the superconducting oxide particles or grains tend to have anisotropic superconducting properties. Oxide superconductors have a multilayered crystal structure, and current tends to flow preferentially within the layers. Sintered ceramic oxide superconducting bodies tend to have randomized grain orientations, and so their current-carrying abilities are reduced to a fraction of the theoretical by the randomly oriented anisotropic grains. Moreover, the grain boundaries between the sintered grains also tend to be poor conductors, further limiting the current flow in a sintered ceramic oxide superconductor.
There is therefore a need for a ceramic oxide superconducting conduit having increased flexibility, homogeniety of properties, no grain boundary conductivity barriers. The present invention is addresses this need.
The present invention relates a flowable, formable high-temperature superconducting slurry. One form of the present invention includes a superconducting slurry formed of substantially spherical high-temperature superconducting oxide powder suspended in liquid nitrogen.
One object of the present invention is to provide an improved magnetic shielding apparatus. Related objects and advantages of the present invention will be apparent from the following description.